I. Field of the Invention
This invention relates generally to monitoring of electrical loads to detect an abnormal load magnitude. In particular this invention relates to monitoring of power delivered to plural electrical heating elements to detect an abnormal load magnitude of any one of the elements.
II. Description of the Prior Art
It is known to use individually controlled electrical heating bands to effect heating in apparatus for extrusion of plastic materials. Such heating bands may be used in conjunction with, for example, extruder barrel segments wherein material is plasticized and extruder outlet elements through which plasticized material is passed as it exits the extruder. It is known to monitor current delivered to the heating bands to detect abnormal operation thereof. In particular, it is known to use current transformers to monitor the current delivered to the electrical heaters. However, in instances involving relatively large total currents, it has been necessary to provide plural current transformers, each transformer being associated with a particular heating zone. Such known techniques do not provide monitoring of the applied voltage and are therefore susceptible of false determinations of abnormal heater operation in the event of applied voltage fluctuations.